Conventionally, the optical touch device first emits, by a light emitting unit thereof, a light source to illuminate a user's hand part, capturing, by an image sensing device thereof, an image of the hand part reflecting the light source, and calculate the gravity center of the hand part image (specifically, the palm part) in the captured image. Thus, according to the gravity center calculated by the optical touch device, the rear end circuit can perform a corresponding operation, for example, controlling a movement of a cursor on a screen.
However, if the captured image includes the user's hand part as well as the user's arm part, the conventional optical touch device may calculate the gravity center based on both of the hand part as well as the arm part, which consequentially may lead to the rear end circuit have an wrong operation, such as moving the cursor to a wrong position on the screen.